Polyvinylidene fluoride membranes which have, in particular, an excellent heat stability and stability towards chemicals are known to be hydrophobic and are difficult to use for separation of aqueous solutions. Numerous attempts to render such membranes hydrophilic by the most diverse types of modification have already been made in the prior art.
German Offenlegungsschrift No. 27 35 887 thus describes a process in which the pores of a porous fluorohydrocarbon polymer are impregnated with at least one water-soluble polymer including polyvinyl alcohol, and the polyvinyl alcohol is rendered insoluble in water by heat treatment or ionizing radiation.
However, impregnation processes have the disadvantage that the membrane structure can become partly blocked by the coating on the membrane pores, which has an adverse influence on the flow values of the membranes. Furthermore, a coating of a hydrophilic polymer on polyvinylidene fluoride is not very stable, especially in the situation of incompatibility of the substrate and coating. It can therefore be destroyed by certain media, in particular sulfuric acid or hypochlorite solutions, which are required for cleaning membranes or in the semiconductor industry, resulting in disadvantages such as increased release of foreign substances or particles, and the hydrophilic character of the membranes is irretrievably lost.
The compatibility of PVDF with hydrophilic polymers is unfortunately very limited. Thus, hydrophilic membranes with a sufficiently high weight content of the hydrophilic component can indeed be prepared from PVDF and a few hydrophilic polymers, such as, for example, polyvinylpyrrolidone. However, they have a very low mechanical strength and the hydrophilic polymer is often extracted from the membrane under use conditions.
PVDF membranes can also be rendered hydrophilic by grafting polymerization. European Patent Application No. 245,000 thus describes a process in which PVDF membranes are first treated with an alkali metal hydroxide solution in order to produce reactive areas on the surface of the PVDF membrane by splitting off hydrogen fluoride Polymerizable hydrophilic vinyl polymers, such as acrylic acid, methacrylic acid and itaconic acid, are then grafted onto these areas using a polymerization initiator. As well as the risk of damage to the membrane by the alkali metal hydroxide solution and blockage of the pores by the polymerized hydrophilic vinyl polymer, this procedure also has the disadvantage that the membrane still contains toxic acrylic monomers and oligomers after the grafting and these can be extracted completely from the membrane only with great expenditure
Another way of avoiding the disadvantages described has been disclosed, according to which a membrane is prepared from a homogeneous mixture of PVDF and a second polymer which can be rendered hydrophilic by chemical reaction, which thus must be compatible with PVDF in the weight range used, and the second polymer is then converted into a hydrophilic polymer by chemical reaction. In European Patent Application No. 012,557, a membrane is thus prepared from a homogeneous mixture of PVDF and polyvinyl acetate and the latter is then hydrolyzed, resulting in a hydrophilic membrane which contains hydroxyl groups because of the polyvinyl alcohol formed This patent application emphasizes, however, that the membranes must contain at least 35 percent by weight of polyvinyl acetate if they are to have an adequately hydrophilic character after the hydrolysis has been carried out. A polyvinyl alcohol content of 43 to 67 percent by weight in the hydrophilic membrane is therefore preferred, which corresponds to a polyvinyl acetate weight content originally present of 60-80 percent by weight However, the favorable polymer properties of PVDF described above are of course worsened considerably by such high weight contents of polyvinyl alcohol or polyvinyl acetate.